Option `per-file-ignores` was introduced in flake8 version 3.7.0.
It allows to ignore specific errors in specific files using glob syntax.
For example `per-file-ignores = src/generated/*.py:F401` will
ignore `F401` error in all python files in `src/generated`.
Thus ale has to run flake8 from project root where .flake8 config
is placed otherwise glob won't match linted file.
